The paper's thesis was simple: when you release a droplet of dichloromethane (DCM) solvent into a beaker of soapy water, it looks really, really cool.
...
To set up the experiment, Steinbock and his fellow researchers filled several beakers with various concentrations of water and a common lab
disinfectant called CTAB. Using a pipette, they added a single drop of DCM — a colorless liquid sometimes used as a degreaser — to each beaker,
and filmed the results. Each trial took about 20-30 seconds total, and was visible with the naked eye.
Each drop of DCM, which has a relatively low boiling point, began evaporating as soon as it left the pipette. But the surprises began when the
droplets touched the soapy water solution.
"DCM has a higher density than water, so you'd expect it to sink right away," Steinbock told Live Science. "But instead, as soon as it touches the
water, part of it spreads out and creates this sort of film that holds the droplet on the surface of the water… it’s like a boat that holds the
droplet afloat.”
...
Despite this boat-like film, a small part of the droplet does begin to sink. It's not visible from the top-down vantage point of this GIF; however, a
tiny jet of falling bubbles forms underneath the droplet once it touches the water. The falling DCM jet slowly shrinks the droplet's volume, but also
causes it to spin. "It’s a little bit like when you flush a toilet," Steinbock said. "The water has a tendency to start to rotate and twist. And
that triggers the rotation of the drop that we begin to see."
Within a few seconds, the droplet is at once floating, rotating and evaporating. As a result of these combined forces, smaller droplets eventually
start disengaging from the edge of the larger droplet. But instead of sinking themselves, they shoot out radially, moving straight ahead over the
surface of the film until they, themselves, evaporate.
"These droplets are self-propelled," Steinbock said. This is due to a phenomenon called the Marangoni effect, which states that a liquid with a high
surface tension will pull more strongly than a liquid with a low surface tension. This difference in tension creates a force on the system that can
lead to motion.
As the DCM in the experiment begins to evaporate, the droplet's surface tension lowers from the outside in. Smaller droplets begin to form at the
large droplet's edge, until the relatively high surface tension of the surrounding water pulls the small droplets away in what Steinbock calls a
"ballistic" trajectory. Each individual droplet moves straight ahead until its surface tension becomes equally unstable, leading to further
fragmentation. Eventually, the droplets split so many times they can no longer be seen. |
